The present invention relates generally to a dynamic behavior control apparatus for an automotive vehicle, and more specifically, to a method and apparatus for controlling the tire side slip angle of the vehicle by controlling the road wheel steering direction of the vehicle.
Dynamic control systems for automotive vehicles have recently begun to be offered on various products. Antilock braking systems (ABS) and traction control (TC) control tire slip ratios. Other dynamic control systems control the side slip angle and yaw response of the vehicle by controlling the tractive forces and/or braking torque at the various wheels of the vehicle. Side slip and yaw control systems typically compare the desired direction, yaw response and lateral acceleration of the vehicle based upon the steering wheel angle, speed, and the direction of travel to an ideal (stable) vehicle model. By regulating the amount of braking torque and tractive friction at each corner of the vehicle, the desired path of travel and yaw rate may be maintained.
When operating the vehicle a large lateral slip angle can occur at the front wheels during severe understeer and oversteer events. The lateral force generated from a tire typically reaches a maximum value Flatmax the slip angle referred to as the slip angle at peak lateral force. The maximum lateral force then decreases or levels off as the slip angle increases further. The leveling off is commonly referred to as the saturation region. One problem with using brake effort to control the lateral dynamics and response of the vehicle is that the yaw moment is controlled without direct control of the lateral forces. This direct control cannot be accomplished using a braking system. However, lateral forces can be controlling using the vehicle steering system.
In particular, it would be desirable to provide a system that allows the lateral dynamics and response yaw of the vehicle to be controlled near the maximum lateral force, to maintain maximum control of the vehicle.
The present invention utilizes a steering system such as a steer-by-wire system that can change the relationship of the road wheel angle to the steering wheel angle. One goal would be to operate close to a maximum lateral force Flatmax, when the vehicle is operated near the limits of its lateral dynamics.
In one aspect of the invention, a stability control system for an automotive vehicle includes a plurality of sensors sensing the dynamic conditions of the vehicle. A controller is coupled to the sensors. The controller determines an estimated road surface coefficient of friction, calculates the maximum lateral force slip angle of the tire xcex1mp, based on the estimated road surface coefficient of friction and vehicle configuration, determines a calculated side slip angle xcex1 in response to measured dynamic vehicle conditions, and reduces a steering wheel actuator angle when the calculated side slip angle is greater than the maximum tire slip angle, xcex1mp.
In a second aspect of the invention, a stability control system for an automotive vehicle includes a plurality of sensors sensing the dynamic conditions of the vehicle. The controller is coupled to the sensors. The controller determines a lateral force in response to measured vehicle conditions, determines a slip angle in response to measured vehicle conditions, determines a first steering actuator angle change, and, in the case of lateral saturation (xcex1 greater than xcex1p) decreases the slip angle until the lateral force increases. This peak-seeking approach searches for the optimal slip and, determines a second steering actuator angle change to increase the slip angle until the lateral force decreases, then iterates upon Flatmax, hence xcex1p.
In a third aspect of the invention, a method of controlling a vehicle having a steering actuator comprises determining a road surface coefficient of friction; calculating a maximum permissible slip angle based on the road surface coefficient of friction; determining a calculated side slip angle in response to measured dynamic vehicle conditions; and reducing a steering wheel actuator angle when the calculated side slip angle is greater than the maximum permissible slip angle.
In a fourth aspect of the invention, a method of controlling a vehicle having a steering actuator comprises determining a lateral force in response to measured vehicle conditions; determining a slip angle in response to measured vehicle conditions; determining a first steering actuator angle change to decrease the slip angle until the lateral force increases; controlling the steering actuator in response to the first steering actuator change angle; thereafter, determining a second steering actuator angle change to increase the slip angle until the lateral force decreases; and controlling the steering actuator in response to the second steering actuator change angle.
One advantage of the invention is that such systems may be easily implemented into a steer-by-wire system. Another advantage is that the slip angle corresponding to the peak lateral force is independent of tire, loading, and in some cases of the surface coefficient of friction.